1. Field of the Invention
This invention relates in general to vacuum heat treating furnaces, and in particular, to a sealing mechanism for a cooling fan drive shaft that penetrates the wall of a vacuum heat treating furnace.
2. Description of the Related Art
Many of the known vacuum heat treating furnaces have an internal gas quenching system. The gas quenching system includes an internal fan for circulating an inert cooling gas over the heated metal parts and through an internal heat exchanger. Commercially available embodiments of such furnaces also have an internally mounted electric motor for driving the gas circulation fan. An example of such a furnace is that sold under the registered trademark “TURBO TREATER” by Ipsen Inc., the assignee of the present application.
The interior of a vacuum heat treating furnace is subject to extreme temperature and pressure conditions. Depending on the type of material being heat treated, the interior of the furnace can reach a temperature of up to 3000° F. (1650° C.), be evacuated to a vacuum of down to about 10−5 torr, and be backfilled with inert gas up to a pressure of up to about 12 bar (1.2 MPa). Under such operating conditions, the useful life of most electric motors is severely curtailed resulting in costly maintenance, repair, or replacement, and furnace downtime. Although the construction of the electric motors used in the known vacuum heat treating furnaces has been modified in various ways to overcome the problems associated with the extreme conditions encountered in such furnaces, none of the modifications have proven entirely satisfactory. The design modifications that work best are also the most expensive to implement. Lower cost modifications have not provided a reliable solution to the problem.
A desirable alternative to locating the fan drive motor inside the furnace vessel is to locate the motor outside the furnace where it is not subject to the temperature and pressure extremes encountered inside the furnace vessel. However, in order to locate the fan drive motor outside the furnace vessel, it is necessary to provide a seal where the drive shaft penetrates the furnace wall. The problem is to effectively provide a vacuum-tight seal for a vacuum as low as about 10−5 torr, as well as to provide a gas-tight seal that is capable of sealing against a fluid pressure of up to 12 bar (1.2 MPa) or higher.
One solution to the foregoing problem is described in U.S. Pat. No. 5,709,544, the entire disclosure of which is incorporated herein by reference. The '544 patent describes a dual seal arrangement that includes an inflatable seal and a lip seal that surround the fan drive shaft where the shaft passes through the furnace wall. The inflatable seal provides a vacuum-tight seal around the drive shaft when inflated. The lip seal provides a gas-tight seal around the drive shaft when the vacuum furnace is pressurized with a cooling fluid and the fan is being rotated. The dual-seal described in the '544 patent has proved effective. However, the lip-type gas seal is a contacting seal and thus, is subject to wear when the drive shaft rotates in operation. In order to avoid premature wearing of the lip seal, some users have limited the rotational speed of the drive shaft. Although the shaft speed reduction benefits the service life of the lip seal, it adversely affects the cooling efficiency of the fan. Another drawback of the lip seal is that the higher the cooling gas pressure used, the greater the force on the lip seal against the drive shaft. The higher sealing force increases the wear rate of the lip seal. Therefore, it has also been necessary to limit the pressure of the cooling gas in order to avoid premature wearing of the lip seal. Although the use of reduced gas pressure benefits the service life of the lip seal, it adversely affects the efficiency of cooling a work load in the furnace.
In addition to the foregoing drawbacks, the dual seal described in the '544 patent includes numerous components which are installed and assembled in place. Maintenance of the seals required disassembling and then re-assembling the seals and the hardware that supports them in the vacuum furnace. Consequently, when it is necessary to perform maintenance on the seals, the furnace has to be shut down for an extended period of time. Extended shut-down periods are highly undesirable in production manufacturing facilities.